Polyamide resin compositions reinforced with fibrous reinforcing materials such as glass fiber and carbon fiber or inorganic fillers such as calcium carbonate are widely known. However, since these reinforcing materials have poor affinity to polyamide, the reinforced polyamide resin compositions have reduced toughness while mechanical strength and heat resistance are improved. Furthermore, molded articles of a fiber-reinforced polyamide resin composition suffer from appreciable warp. Additionally, in using the inorganic fillers, substantial improvement in mechanical strength or heat resistance cannot be obtained unless they are added in a large quantity.
In order to eliminate these disadvantages of conventional reinforced polyamide resin compositions, a composition comprising polyamide and a layered silicate typified by montmorillonite has been proposed (unexamined published Japanese patents No.62-74957, No.63-230766, No.2-102261, and No.3-7729). In this resin composition, silicate layers are uniformly dispersed in the polyamide resin on the molecular level by introducing polyamide chains into the laminae of a layered silicate. In the case of using montmorillonite for the above purpose, the montmorillonite must be swelled with organic salts (e.g., an ammonium salt of an aminocarboxylic acid, an onium salt) and then isolated, before the monomer or monomers forming the polyamide resin are polymerized.
Meanwhile, the present inventors discovered that polyamide resin compositions having a specific layered silicate (swellable fluoromica mineral) uniformly dispersed therein on the molecular level and exhibiting excellent mechanical strength and heat resistance can be obtained without the above-described inorganic salt treatment. This is achieved by mixing monomer(s) forming polyamide and swellable fluoromica mineral, and polymerizing the above-described monomer(s) usually under a high pressure of over 10 kg/cm.sup.2 (unexamined published Japanese patent No. 6-248176).
Furthermore, the present inventors have proposed a process which comprises mixing monomer(s) forming polyamide, swellable fluoromica mineral, and an acid having pKa of 0.about.6 or negative (in 25.degree. C. water), and then mixing the above-described monomer(s) (unexamined published Japanese patents No.8-3310 and No.8-134205). Because this process does not necessarily require high pressure during polymerization, polymerization usually under a pressure of about 5 kg/cm.sup.2 provides a polyamide resin composition which provides molded articles exhibiting excellent mechanical strength, toughness, heat resistance, and dimensional stability, low water absorption ratio, and an improved water absorption property including no decrease of the above-described properties under the influence of water.
However, for the above-described polyamide resin composition, the tensile strength of molded articles is always insufficient and the deviation of the values is substantially large, being independent of the kind of layered silicates. Moreover, when manufacturing the above-described polyamide resin composition on an industrial scale, some problems arise from the viewpoint of productivity including observation of increased pressure at a nozzle part at the withdrawal of the polymer and consequently frequent exchange of filters.
As a result of extensive studies, the present inventors have discovered that the above problems are caused by an insufficient degree of uniform dispersion of a layered silicate into a nylon matrix on the molecular level, and at the same time by oversized particles which remain in the resin composition originating from impurities contained in a layered silicate as a raw material which did not participate in the above uniform dispersion.
This invention provides a polyamide resin composition which provides molded articles exhibiting high strength, high modulus, high heat resistance, high toughness, excellent dimensional stability, and high tensile elongation with small deviation. Also provided is a process for producing said polyamide resin composition.